Field of the Invention
The invention relates to systems and methods for determining the properties of materials, and more particularly, to systems and methods that apply a pressure to a material and analyze a three-dimensional topography of the resulting surface deformation to determine mechanical properties of the material. The invention also relates to systems and methods that apply a pressure in-vivo to human tissue and analyze a three-dimensional topography of the resulting surface deformation to identify localized inhomogeneities and anomalies in the human tissue.
Description of Related Art
It is often desirable to know the mechanical properties (e.g., stiffness, hardness, viscoelasticity, directional anisotropy) of materials. Both quasistatic and dynamic aspects of these properties are of interest. Such materials may be man-made (e.g., metal, plastic, fabric) or natural (e.g., human tissues, animal tissues, plant matter). Sometimes a determination of the bulk average of a mechanical property is sought. At other times, it is important to know the local mechanical properties, especially when the material is known to be inhomogeneous.
There are many well-established approaches for determining the mechanical properties of materials. With an assumption of homogeneity, bulk mechanical properties can be determined from measurements of overall loads and deflections imparted to a sample of the material. Another approach for measuring certain mechanical properties is to apply a known pressure to the surface of a material and measure the resulting deformation at the surface. This is a common method for measuring the hardness of metals.
For soft, or low modulus, materials (e.g., certain plastics, foams, biological soft tissues), referred to here as deformable materials, it is sometimes inconvenient or impossible to grasp the bulk material for the purpose of imparting loads or deformations. It is also sometimes desirable to make measurements without directly touching the material with instruments. In-vivo measurement of soft tissue properties is such a case where removing material for testing, or contacting the tissue with instruments, can be injurious.
In addition to measuring bulk material properties in deformable materials, it is sometimes desirable to locate and characterize highly localized inhomogeneities or anomalies within the bulk material. For example, there may be a flaw in the material that is markedly harder or softer than the surrounding material. In the case of biological soft tissues, a locally diseased area (e.g., a cancerous nodule) is often harder than the healthy tissue around it. Indeed, the localized differences in mechanical properties of healthy and diseased tissue form the basis of the centuries-old practice of palpating (i.e., touching and manipulating) tissue to diagnose and treat patients.
A particularly challenging situation in medicine is measuring in-vivo tissue mechanical properties during minimally invasive surgery (MIS) such as laparoscopy, arthroscopy, and thoracoscopy. In MIS, the surgeon accesses and views the target anatomy through small incisions using long, small-diameter instruments and optics. It is difficult or impossible to directly palpate tissues with the fingers, making it necessary to use specially designed instruments to gain an understanding of tissue mechanical properties and to search for anomalies in the tissue. In one type of MIS, thoracoscopic surgeons remove small cancerous nodules from lung tissue that have been identified using CT scans. However, if the target nodule is below the surface of the lung, locating it can be very difficult using standard MIS instruments.
Approaches for measuring bulk and local material properties of biological soft tissues, and for locating and differentiating diseased from healthy areas of tissue, include use of sonic and ultrasonic wave propagation in tissue. These approaches are not optimal, especially for use in MIS applications, either because the apparatus is too large to conveniently be deployed through small openings or because the instrument must directly contact the tissue in order to make the measurement. In addition, these approaches are often time consuming to carry out, making them impractical for use during surgery.